This application claims the benefit of Korean Patent Application No. 2003-40085, filed on Jun. 20, 2003, in the Korean Intellectual Property Office, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to lithium batteries generally. More particularly, the present invention relates to an anode composition for a lithium battery that can improve anode characteristics and battery characteristics while using water as a solvent, and to an anode and lithium battery using the same.
2. Description of the Related Art
Recently, as portable electronic devices such as personal data assistants (PDAs) cellular phones, and notebook computers have become widely used in various areas, batteries for driving these electronic devices have become smaller, thinner, and lighter, and offer improved performance.
Due to advantages such as light weight and high energy density, lithium batteries are used as main driving sources for many portable electronic devices. Like all batteries, lithium batteries have an anode and a cathode, each formed of an electrically active material. The active material used to form cathodes (hereinafter, cathode active material) may be a lithium-containing transition metal oxide such as LiCoO2 and LiNiO2 or a chalcogen compound such as MoS2. These compounds allow reversible intercalation or de-intercalation of lithium ions because of their layered crystalline structure, and thus, have been widely used as a cathode active material in lithium batteries.
An active material used to form anodes (hereinafter, anode active material) in lithium batteries is lithium metal. However, when lithium metal is subjected to intercalation and de-intercalation during a charge/discharge cycle of lithium batteries, needle-shaped lithium dendrites are sometimes repeatedly precipitated on the surface of the anode. These needle-shaped lithium dendrites may not only decrease the battery's charge/discharge efficiency but also can contact a cathode, thereby causing an internal short-circuit.
In view of these problems, alternative anode active materials have been suggested. Examples include: a lithium alloy, a metal powder, a carbonaceous material such as a graphite, a metal oxide, or a metal sulfide that enables reversible lithium intercalation/de-intercalation. However, when a charge/discharge cycle is repeated in lithium batteries that use a lithium alloy sheet as an anode, the efficiency of current collection may be lowered due to pulverization of the alloy sheet, thereby deteriorating the battery's charge/discharge cycle characteristics.
Due to these disadvantages, a sheet anode cannot be formed solely of a metal powder, a carbonaceous material, a metal oxide, or a metal sulfide. Thus, a binder is generally added. For example, an operable anode may be made of a mixture of a carbonaceous anode active material and an elastic, rubber-based, polymer binder.
When metal oxide or metal sulfide is used as a base anode active material, a conductive material can be used in addition to a binder to enhance the battery's charge/discharge characteristics. Generally, a carbonaceous material for an anode is crushed into powder and mixed with a binder to form an anode plate. Then a rubber-based polymer is used as a binder to coat the carbonaceous material particles, but this renders the intercalation and de-intercalation reaction of lithium ions difficult. As a result, the high efficiency discharge characteristics of lithium batteries are remarkably reduced.
Another drawback is that use of only a conventional binder in the absence of other additives provides poor adhesion between a carbonaceous material and a metal substrate made of a metal, regardless of the type and shape of the carbonaceous material used. To compensate, a large amount of a binder is required. The carbonaceous material may be covered with the binder, but this decreases the battery's high efficiency discharge characteristics. On the other hand, if the binder is used in a small amount to maintain discharge characteristics, an anode active material layer may be separated from the substrate. However, such a configuration renders formation of a sheet anode difficult and increases the chances of forming a poor anode plate. In this regard, attention has been focused on the search for an alternative method to increase adhesion between a carbonaceous anode active material and a substrate while avoiding excess use of a binder in lithium batteries. One previously-attempted solution discloses a mixed binder for an anode including a polyamic acid and at least one polymer selected from the group consisting of a polyamide resin, polyvinylpyrrolidone, and hydroxyalkylcellulose to ensure a long lifecycle and enhance reliability.
However, since the polyamic acid for the mixed binder must be removed by thermal treatment at 200 to 400° C. during drying of the anode plate, the manufacturing process is complicated and the physical properties of the anode may change during manufacture. Thus, a mixed binder of polyvinylpyrrolidone and styrene butadiene rubber (SBR) has been suggested as an alternative binder material for an anode. However, use of the mixed binder may lower uniformity of the anode due to an adhesion strength difference between the two materials, and cause separation of an anode active material during a charge/discharge cycle or make solid components loose.
Additionally, binders for manufacture of an anode composition usually need an organic solvent such as N-methyl-2-pyrrolidone (NMP) that is harmful to humans. Therefore, there arise problems in that the anode manufacturing process is complicated, because multi-step processes and apparatuses are required, and because environmental contamination due to emission of contaminants, such as organic solvents, occurs. In an attempt to solve these problems, one method proposed preparing a slurry of aqueous anode active material, the slurry containing water as a solvent together with a water-soluble SBR binder. A drawback of this approach, however, is that use of a small quantity of only the SBR based binder may cause a decrease in adhesion and anode characteristics as described above, because the SBR binder has weak adhesion due to its point contact adhesive properties and small contact area with the active material. Consequently, use of only an SBR binder may cause separation of the active material from an electrode plate or decrease an adhesion between active materials. Thus, use of only a SBR binder may decrease the charge/discharge cycle characteristics of lithium batteries.